Having said that, page language selection is working correctly on testwiki, so I've scheduled the config change (matching what is proposed in the task description) for the second-last SWAT of the year tomorrow morning.

What is Special:PageLanguage suposed to do? Any link or explanation would
be helpful, even if it seems obvious to all of you. Thank you :-)

It allows you to specify the content language of an individual page. This then tells the Translate system which language is the "source" language for translations of the page. Currently, the wiki's default language is the only possible source language.

From an on-wiki perspective, nothing will change unless community members actively decide to make use of this feature. Potential for abuse is essentially nil, as the right to change page languages will only be available to admins and translation administrators. What's more, it will only be available on wikis with the Translate extension installed.